Optoelectronic oscillators (OEOs) provide the combined lowest phase noise and widest bandwidth signal sources currently available in the microwave and millimeter frequency ranges. Commercial units are becoming available which are setting a new standard with respect to system phase noise and, as a result, system sensitivity.
The improved phase noise performance provided by OEOs allows improved RADAR detection. Previously, travelling wave tube amplifiers (TWTAs) had limited the RADAR integrated phase noise, known as coherence, to levels of approximately −50 dBc over integration bandwidths from 10 Hz to 1 MHz. The use of solid state amplifiers provided improvements in the coherence of the microwave power generation device to approximately −70 dBc, at which point the local oscillator phase noise became the limiting factor with respect to RADAR coherence. OEOs enable −90 dBc noise floors, which moves the system coherence limitation back to other components in the RADAR.
In order to obtain OEO oscillations, a commonly used component is an optical delay line. The OEO phase noise decreases as the length of this optical delay line is increased. For a given optical delay line length, an OEO will produce periodic oscillatory modes resulting in an output frequency comb. For use as an oscillator signal in a radio frequency system, a single mode, or frequency, is desirable. Mode selection can be made by a bandpass filter of suitable center frequency and bandwidth.
It is desirable to have a long optical delay line length to obtain low phase noise. However, as the delay line is increased, the frequency spacing between adjacent modes decreases. For example, for a RADAR oscillator signal of 10 GHz, the mode spacing for desired OEO phase noise performance could be 400 kHz. Producing a bandpass filter centered at 10 GHz with a bandwidth of less than 400 kHz bandwidth is not possible with current OEO topologies.
Some approaches have used multiple parallel optical delay lines of different lengths to increase the mode spacing to approximately 30 MHz. This allows the use of YIG filters, which can be tuned. However, YIG filters are bulky, expensive and have slow tuning characteristics. Multiple optical delay lines require additional photodetectors and optical modulator input ports. Additional problems with using multiple parallel optical delay lines of different lengths include: increased sensitivity to thermal drift and mechanical vibration; and an increase in size due to the additional bulk resulting from the multiple parallel optical delay lines.
Other attempted solutions have used glass whispering gallery mode resonators. These have the advantage of smaller size than optical fiber delay line based solutions. However the glass resonator technology is not readily manufacturable, resulting in higher costs and lower production volumes.
To realize a wide bandwidth low phase noise OEO there is a need for a filter that can operate at the RADAR oscillator frequency, which is able to filter tightly spaced OEO modes, and is capable of fast and fine frequency tuning.